1. Field of the Invention
The present invention relates to a flexible sheet-type piezoelectric ceramics-polymer composite material. Also, the present invention is concerned with a method for the preparation of the same.
2. Description of the Prior Art
Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 (hereinafter referred to as "PZT"), widely used as a representative piezoelectric ceramic material, exhibits exceptionally high piezoelectric effects at morphotropic phase boundary (hereinafter referred to as "MPB") which is generated when the composition ratio of Zr to Ti is 52:48 in PZT, at which point PZT alternates between tetragonal phase and rhombohedral phase. In such composition, PZT is maximum in electromechanical coupling coefficient (K.sub.p, K.sub.t), piezoelectric charge coefficient (d) and dielectric constant (.epsilon..sub.r).
Aside from its remarkable piezoelectric effect, PZT is an excellent piezoelectric material because it has no transition temperature in a temperature range of -50.degree. to 200.degree. C. therein. Accordingly, active research and development efforts have been directed to novel PZT piezoelectric materials.
Currently, PZT ceramics have been useful for a plurality of purposes including electromagnetic articles such as a transducer for medical diagnosis, an underwater sound detector, a buzzer, an ignition device and the like. The PZT ceramics have, however, a limit in application for fine medical diagnosis since it exhibits a sound impedance of 30 Mrayl which is considerably high relative to human skin showing a sound impedance of 1.5 Mrayl.
Meanwhile, most piezoelectric ceramics were bulk type. Such bulk-type piezoelectric ceramics have much difficulty in application for electromagnetic articles which have recently been on a trend of simplification, thinness, shortness and smallness. Accordingly, there have been studied for piezoelectric bodies of thick-film type or sheet type instead of bulk type. For the preparation of such sheet or thick-film type piezoelectric bodies, a polymeric material is used together with piezoelectric ceramics. Preferably, the polymeric material has superior molding processability and flexibility, and is easy to make large in area and possible to produce in large quantities.
Polyvinylidine fluoride (hereinafter referred to as "PVdF") and the copolymers thereof (e.g. TrFE, TFE) are useful for piezoelectric materials. PVdF, which has a sound impedance of 4 Mrayl is intensively used for transducers requiring especially low sound impedance. However, there are pointed out disadvantages in PVdF that it is low in electromechanical coupling coefficient (K.sub.t .perspectiveto.0.3), small in dielectric constant (.epsilon..sub.r .perspectiveto.10), and has high dielectric loss relative to piezoelectric ceramics.
In addition, not only does PVdF polymer poorly bind with ceramics, but pores are generated at the interfaces therebetween, which results in difficulty of producing a highly dense, precise piezoelectric material.
Making the best of the respective advantages of PZT ceramics and PVdF piezoelectric polymeric material, piezoelectric composite materials have been manufactured in a bulk type of pellet shape or in a sheet type with a thickness of several hundreds .mu.m. However, these materials have many problems as follows.
On the one hand, the pellet-shaped bulk type piezoelectric composite material, lacks flexibility and is difficult to make large in area, like the bulk type utilizing pure piezoelectric ceramics, so that advantage cannot be taken of molding processability, a property of polymer. On the other hand, in the sheet type piezoelectric composite material, the adhesion of PZT to PVdF is neither uniform nor precise, resulting in the formation of pores at the interfaces between PZT and PVdF. Accordingly, in case of the sheet type piezoelectric composite material with a thickness of several hundreds .mu.m, it is difficult to obtain a highly dense, precise composite material and also has deleterious effects in properties.